Fix #771

Author: halx99

core/renderer/backend/metal/CommandBufferMTL.mm

@@ -207,7 +207,7 @@ inline int clamp(int value, int min, int max)
         _currentRenderTargetFlags == renderTarget->getTargetFlags() &&
         !renderTarget->isDirty())
     {
-        return _mtlRenderEncoder;
+        return;
     }
 
     _currentRenderTarget      = renderTarget;